In a MEMS capacitive sensor, for example a MEMS microphone, a capacitive profile of the sensor is changed by moving a membrane with respect to a backplate to convert an acoustical input signal into an electrical output signal of the capacitive sensor. The membrane and the backplate of the sensor form a capacitor having a variable capacitance that is changed in dependence on the distance between the membrane and the backplate of the capacitor. If the capacitive sensor is configured as a MEMS microphone, the capacitance of the capacitor of the sensor is changed in dependence on the acoustical pressure of an acoustical signal effecting on the membrane of the capacitor.
A bias voltage generator and a filter circuit are connected to a bias voltage input terminal to provide a bias voltage to the capacitive sensor. The filter provides a ripple free and noiseless bias voltage for operating the capacitive sensor. The capacitive sensor generates an output signal at an output terminal of the capacitive sensor in dependence on the distance between the membrane and the backplate of the capacitor.
The output signal generated at the output terminal of the capacitive sensor is usually amplified by an amplifier circuit comprising a capacitive sensing amplifier. The amplifier circuit is coupled to the output terminal of the capacitive sensor to receive and amplify the output signal of the capacitive sensor. In order to set an operation point of the amplifier of the amplifier circuit, a DC level of the amplifier is set to a predefined value by means of a control circuit in a feedback loop/DC servo loop. The control circuit in the feedback loop controls the input DC level of the amplifier.
A startup time of the arrangement comprising the bias voltage generator/filter circuit for providing the bias voltage, the capacitive sensor and the amplifier circuit is defined as the time until the output signal of the amplifier circuit is ready to use. The time it takes for the output to settle to its final value is referred to as the settling phase. After the arrangement has started up, the output signal if no signal is applied to the sensor has to stay within pre-defined limits.
In order to bring the capacitive sensor into a status ready for operation a bias voltage has to be applied to a bias voltage input terminal of the capacitive sensor to provide a charge at the plates of the capacitor of the capacitive sensor. After the capacitive sensor is ready for operation, the bias voltage provided at the bias voltage input terminal is settling to its final level. During the settling process, charge will be transferred by the capacitor of the capacitive sensor to the input terminal of the amplifier circuit. If the charge at the plates of the capacitor of the capacitive sensor remains constant after the settling of the bias voltage no more charge is transferred to the input terminal of the amplifier circuit. The settling of the bias voltage is influenced by the start-up characteristics of the voltage generator/filter circuit for providing the bias voltage as well as any leakage characteristics of the capacitive sensor. The settling can take several seconds before the bias voltage reaches its final bias voltage level.
For normal operation the bandwidth of the DC servo loop of the amplifier circuit is set such that it does not interfere with the bandwidth of the output signal of the capacitive sensor. In audio applications using a MEMS microphone as capacitive sensor, the bandwidth of the control circuit of the feedback loop of the amplifier circuit is required to be set in the hertz range leading to settling times in the range of seconds. For quick startup of the device the bandwidth is required to be increased. If the bandwidth of the DC servo loop is too low to compensate for the rate of change of the bias voltage level during settling, the input voltage level at the input terminal of the amplifier of the amplifier circuit will start to deviate from its final DC bias value. On the other hand, larger loop bandwidths of the control circuit of the feedback loop will reduce the bandwidth of the output signal, resulting in an increased noise level of the amplified output signal and leading to an excessive phase delay.
It is a desire to provide an amplifier circuit for amplifying an output signal of a capacitive sensor that allows to sufficiently compensate the rate of change of the bias voltage of the capacitive sensor during the settling phase.
Furthermore, there is a demand to provide a capacitive sensor arrangement comprising an amplifier circuit being configured to sufficiently compensate the rate of change of a bias voltage level of the capacitive sensor during the settling phase.
Furthermore, it is desired to provide a method to amplify an output signal of a capacitive sensor, wherein an amplifier circuit to amplify the output signal of the capacitive sensor can sufficiently compensate the rate of change of a bias voltage level of the capacitive sensor during the settling phase.